The present invention in general relates to an interface circuit used for transmission of logic signals among a plurality of electronic devices. More particularly, this invention relates to an interface circuit and a signal transmission method suitable for use in a plurality of electronic devices having different power supply voltages.
As the technique advances, a plurality of electronic devices such as LSIs (Large Scale Integrated circuits) or ICs (Integrated Circuits) are mounted on each electronic apparatus. By conducting transmission and reception of logic signals between these electronic devices, given operation is executed. Recently, however, power supply voltages of a plurality of electronic devices tend to differ according to manufactures. Therefore, there frequently occurs such a situation that the level of the logic signal also differs among a plurality of electronic devices.
Heretofore, therefore, it has been necessary to adjust the level of the logic signal among a plurality of devices. This results in such a state that evil effects attendant upon the adjustment, such as a longer circuit design time and restriction of usable electronic devices, cannot be overlooked. Heretofore, therefore, a measure and method capable of effectively solving such problems have been desired earnestly.
As an example of the above described electronic apparatus having a plurality of electronic devices, a magnetic disk apparatus shown in FIG. 6 will now be described. FIG. 6 is a block diagram showing the configuration of a conventional magnetic disk apparatus. As shown in FIG. 6, in this magnetic disk apparatus, a HDC (hard disk controller) 1 is connected to a host 9 via a SCSI (Small Computer System Interface) bus which is not illustrated. Between the HDC 1 and the host 9 via the SCSI bus, various commands (such as a read command and write command), write data to be written onto a magnetic disk M described later, and read data read out from the magnetic disk M are transmitted and received. The HDC 1 controls components of the magnetic disk apparatus.
A MPU (Micro Processing Unit) 2 controls components of the apparatus. A read/write control and power save control are the major example of such control. A read channel 3 includes a modulation circuit for writing write data onto the magnetic disk M, a parallel/serial conversion circuit for converting parallel write data to serial data, and a demodulation circuit for reading out read data from the magnetic disk M. In addition, the read channel 3 includes a serial/parallel conversion circuit for converting serial read data to parallel data, and a synthesizer circuit for generating timing signals to be used as timing of components of the apparatus by conducting frequency multiplication on the frequency of an oscillation circuit using a crystal oscillator.
A head IC 4 drives a magnetic head 5. The head IC 4 delivers read data fed from the magnetic head 5 to the read channel 3, and delivers write data fed from the read channel 3 to the magnetic head 5. The magnetic head 5 is disposed in close vicinity to the magnetic disk M. The magnetic head 5 has a function of magnetically writing write data onto the magnetic disk M and a function of magnetically reading out read out from the magnetic disk M.
A SPM (spindle motor) 6 drives and rotates the magnetic disk M. A VCM (voice coil motor) 7 moves the magnetic head 5 in the radial direction of the magnetic disk M. A servo controller 8 effects servo control by controlling the SPM 6 and the VCM 7. The HDC 1, MPU 2, read channel 3, head IC 4, and servo controller 8 are electronic devices such as LSIs and ICs, and driven with different power supply voltages.
Therefore, levels of logic signals transmitted and received between these electronic devices also differ depending on electronic devices. In the conventional magnetic disk apparatus, therefore, the level adjustment of logic signals is conducted by an interface circuit shown in FIG. 7. In FIG. 7, an LSI A and an LSI B correspond to two among the HDC 1, the MPU 2, the read channel 3 and so on shown in FIG. 6. The LSI A is driven by a first power supply voltage Vcc1 (=5.0 V). A logic signal S1 of the LSI A also has a level of 5.0 V.
On the other hand, the LSI B is driven with a second power supply voltage Vcc2 (=2.5 V) which differs from the first power supply voltage Vcc1 (=5.0 V). A logic signal S2 of the LSI B also has a level of 2.5 V. That is, the logic signal S1 of the LSI A has a level of 5.0 V, whereas the logic signal S2 of the LSI B has a level of 2.5 V. If the logic signal S1 of 5.0 V is inputted to the LSI B while leaving this state intact, therefore, the logic signal S1 exceeds the breakdown voltage of the LSI B and there is a possibility of the LSI B being destroyed.
In the conventional interface circuit, therefore, there is provided a level converter C for conducting level adjustment of the logic signal between the LSI A and the LSI B. The level converter C converts the logic signal S1 of 5.0 V to the logic signal S2 of 2.5 V conforming to the LSI B. By virtue of the level adjustment conducted by the level converter C, the LSI A and the LSI B normally function.
Thus, the level converter C for conducting the level adjustment of the logic signals must be used in the conventional technique when a plurality of electronic devices driven with different voltages are to be utilized. Therefore, it is necessary to select an electronic device which conforms to the level conversion characteristic of the level converter C. Even if an electronic device is high in performance as a simple substance, therefore, the electronic device cannot be used in the electronic apparatus so long as it does not conform to the level conversion characteristic, in the case of the conventional technique. Therefore, usable electronic devices are restricted naturally. This results in a problem that the circuit design is severely restricted.
Furthermore, if the level converter C is not used, there is a restriction that a plurality of electronic devices which are equal in logic signal level must be selected. This results in a problem that the circuit design time becomes longer by the time required for selection.
It is an object of the present invention to provide an interface circuit and signal transmission method capable of increasing the degree of freedom of the circuit design using a plurality of electronic devices driven with different voltages and shortening the circuit design time.
In the present invention, a constant current control unit outputs a constant current to the second electronic device, and a logic signal generation unit generates a logic signal having a level conforming to the second electronic device on the basis of the constant current. Thus, logic signal transmission using the constant current is conducted between the first electronic device and the second electronic device, and a logic signal having a level conforming to the second electronic device is generated on the basis of the constant current, in the second electronic device. Accordingly, even if a plurality of electronic devices driven with different power supply voltages are present, the level of the logic signal can be easily conformed. As a result, it becomes possible to increase the degree of freedom of the circuit design and shorten the circuit design time.
Other objects and features of this invention will become apparent from the following description with reference to the accompanying drawings.